SWODY1
SPC AC 121247
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0747 AM CDT MON SEP 12 2011
VALID 121300Z - 131200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON THROUGH EARLY
TONIGHT FOR NE MN...NRN WI...AND UPPER MI...
...NE MN/NRN WI/UPPER MI THIS AFTERNOON INTO EARLY TONIGHT...
A MIDLEVEL SHORTWAVE TROUGH OVER SRN SK/MB WILL MOVE QUICKLY ESEWD
OVER NRN MN/WI/UPPER MI BY EARLY TONIGHT. A DEEPENING SURFACE
CYCLONE WILL PRECEDE THE MIDLEVEL TROUGH AND DEVELOP EWD NEAR THE
MN/ONTARIO BORDER THIS AFTERNOON...WHILE A TRAILING COLD FRONT MOVES
SEWD ACROSS THE UPPER MS VALLEY. LOW-LEVEL MOISTURE WILL REMAIN
SOMEWHAT LIMITED IN ADVANCE OF THE FRONT...WITH BOUNDARY LAYER
DEWPOINTS EXPECTED TO REMAIN IN THE 55-60 F RANGE TODAY.
HOWEVER...A PLUME OF STEEP MIDLEVEL LAPSE RATES /NEAR 8 C PER KM/ IS
SPREADING EWD IN ADVANCE OF THE COLD FRONT AND MIDLEVEL TROUGH...AND
WILL COMBINE WITH DAYTIME HEATING TO SUPPORT AFTERNOON MLCAPE VALUES
APPROACHING 1500 J/KG.
THE COMBINATION OF HEATING/MIXING AND A FOCUSED BAND OF ASCENT ALONG
THE FRONT WILL PROMOTE THUNDERSTORM DEVELOPMENT AROUND MID AFTERNOON
INVOF THE MN ARROWHEAD...WITH CONVECTION EXPECTED TO DEVELOP SWD
INTO NRN WI AND SPREAD EWD OVER UPPER MI THROUGH EARLY TONIGHT. THE
LINEAR FORCING FOR ASCENT AND STRONG DEEP-LAYER FLOW/SHEAR VECTORS
ORIENTED ACROSS THE FRONT SHOULD SUPPORT A MIXED CONVECTIVE MODE OF
LINE SEGMENTS AND EMBEDDED SUPERCELLS. THE SOMEWHAT MARGINAL
MOISTURE...STEEP LAPSE RATES...AND TENDENCY FOR LINEAR ORGANIZATION
WILL FAVOR DAMAGING WINDS AND LARGE HAIL AS THE MAIN SEVERE WEATHER
THREATS IN THE 21-03Z TIME FRAME.
...ERN CAROLINAS THIS AFTERNOON...
IN RESPONSE TO UPSTREAM HEIGHT FALLS OVER THE UPPER MS VALLEY...A
MIDLEVEL TROUGH IS NOW MOVING EWD FROM THE SRN/CENTRAL APPALACHIANS
TOWARD THE ATLANTIC COAST. RESIDUAL BOUNDARY LAYER DEWPOINTS IN THE
60S AND A WEAK SURFACE TROUGH ACROSS THE ERN CAROLINAS WILL HELP
FOCUS THUNDERSTORM DEVELOPMENT THIS AFTERNOON IN CONJUNCTION WITH
DAYTIME HEATING. MLCAPE NEAR 1000 J/KG AND STEEP LOW-LEVEL LAPSE
RATES COULD SUPPORT STRONG OUTFLOW GUSTS AND MARGINALLY SEVERE
HAIL...THOUGH WEAK LOW-LEVEL FLOW AND POOR LAPSE RATES ALOFT WILL
TEMPER THESE THREATS.
...AZ THIS AFTERNOON/EVENING...
A PRONOUNCED BELT OF MIDLEVEL MOISTURE EXTENDS NNEWD FROM NRN BAJA
INTO AZ...E OF THE WEAK CLOSED LOW OVER CA. THE MOISTENING ALOFT
COMPARED TO PREVIOUS DAYS WILL TEND TO LIMIT THE MAGNITUDE OF
INSTABILITY THIS AFTERNOON...THOUGH SURFACE HEATING IN CLOUD BREAKS
WILL STILL RESULT IN STEEP LOW-LEVEL LAPSE RATES. INVERTED-V
PROFILES IN THE LOW LEVELS AND MODEST DEEP-LAYER SHEAR COULD SUPPORT
ISOLATED STORM CLUSTERS CAPABLE OF PRODUCING STRONG OUTFLOW GUSTS.
..THOMPSON/HURLBUT.. 09/12/2011
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